Known as conventional navigators are apparatuses that are so configured as to acquire the information of the current location by the use of a current location detecting means, such as a GPS (global positioning system) receiver, then read out map information stored in a storage means, such as a CD-ROM, and then display a map around the current location on a displaying means, such as a monitor, and indicate the current location by the use of a guiding means, such as by displaying it or telling it with voice.
Some navigators can receive the latest, detailed map information and locality information through communication achieved by means of FM multiplex broadcast, known as the VICS (vehicle information and communication system) service, or by means of radio beacons or optical beacons, or by other means.
Other navigators under development incorporate, for compactness and light weight, a compact storage medium, such as a Smart Media card, Compact Flash card, Memory Stick, or SD card, instead of a disk drive, such as a CD-ROM or DVD-ROM drive. Since such compact storage media have small storage capacities, with navigators incorporating them, it is essential to receive map information or locality information through communication in a manner as described above.
For example, Japanese Patent Application Laid-Open No. 2000-67385 discloses a navigator that acquires from a map center only such information as is currently needed and that then displays it. This navigator does not require a memory for the storage of currently unnecessary map information, and keeps acquiring the latest map information.
However, even though information can be acquired through communication in a manner as described above, the navigator has only a small storage capacity, and therefore receiving large-scale, detailed map information and locality information all the time increases the frequency of reception, resulting in a high charge for communication. Moreover, receiving both map information and locality information increases the amount of information, increasing the frequency of reception and thus the charge for communication.
Moreover, even though information can be acquired through communication in a manner as described above, since the navigator has a small storage capacity, it is necessary to receive information periodically. If, when there is a need to receive information, the navigator happens to be located in an area where communication is impossible, as in a tunnel, the information being received is interrupted.
Moreover, according to Japanese Patent Application Laid-Open No. 2000-67385 mentioned above, the map information of not only the map area of the current location but also all the map areas adjacent thereto is kept acquired. Thus, the map information of already traveled locations and such map information as is highly unlikely to be used is also kept acquired unnecessarily. This method, therefore, is too inefficient to be applied to a navigator having a limited storage capacity. Specifically, when the navigator happens to be traveling an area where communication is impossible, the map information being received is highly likely to be interrupted.